In recent years, many attempts have been extensively made to obtain a high-tenacity and high-modulus fiber by use of an ultrahigh-molecular-weight polyethylene as a raw material, and some reports can be found on a polyethylene fiber having very high tenacity and very high elastic modulus. For example, the so-called "gel spinning method" is disclosed in Japanese Patent Laid-open Publication No. 15408/1981. In this method, an ultrahigh-molecular-weight polyethylene is dissolved in a solvent and extruded into a gel fiber which is then stretched at a high ratio.
It is well known that the high-tenacity polyethylene fiber obtained by the "gel spinning method" has very high tenacity, very high elastic modulus and quite excellent impact resistance for an organic fiber. For various purpose, its application has been partly spreading over some technical fields. With the object of producing such a high-tenacity fiber, the above publication further discloses a technique for providing a material having very high tenacity and very high elastic modulus.
The conventional technique is, however, disadvantageous in that an attempt to produce a high-tenacity fiber will contrarily cause deterioration of vibration-absorbing properties at low temperatures as described in detail blow. This disadvantage, therefore, makes it impossible to obtain a high-tenacity fiber having retained vibration-absorbing properties at low temperatures.
For various purposes of protective materials, they have recently been required to have higher impact resistance to meet the needs of their weight saving. Also for the conventional high-tenacity fibers, they have been required to have resistance to higher-speed impact, and there is a great demand for the appearance of an innovative material. There remains a discussion on the factor making a dominant contribution to high-speed impact-absorbing properties, and in the case of a polyethylene fiber which is a viscoelastic material from a rheological point of view, it is predicted from the famous rule of time-temperature superposition that high-speed deformation will receive a great influence from the physical properties of the fiber at low temperatures. Accordingly, if a material having vibration-absorbing properties at low temperatures, more generally speaking, a material absorbing impact energy, can be obtained, such a material may also be used as a material having quite excellent high-speed impact-absorbing properties.